A stepping motor is one which is turned a fixed angle by a pulse derived from a driving unit, and generally applied to a printer, which is installed in a portable terminal, as a driving unit.
The rotor of a stepping motor is rotated by magnetic attraction and magnetic repulsion generated between the rotor and the exciting coils which are wound on the a stator and excited in sequence, and it is feared that an abnormal rotating condition of the rotor, that is, a loss of synchronism is caused by fluctuation of the driving load and/or the voltage. Therefore, it becomes necessary to surely detect a loss of synchronism, because behavior such as disturbed printing is caused when a loss of synchronism occurs.
Hitherto, a constant-voltage exciting system was generally used for exciting the exciting coils of a stepping motor, and a detector to detect a loss of synchronism for constant-voltage exciting system has already been proposed (See Unexamined Patent Application (Kokai) No. 63-87198).
FIG. 1 is a circuit diagram of a detector to detect a loss of synchronism, and one terminal of an exciting coil 11 is grounded through a control transistor 12 and a resistor for measuring a current 13, and the other terminal is connected to a DC bus V.sub.cc. An exciting pulse is applied to the base of the control transistor 12.
The voltage across the resistor for measuring a current 13 is applied to one input terminal of a comparator 14, and is compared with a reference voltage generated by a reference voltage source 15 which is applied to the other input terminal of the comparator 14.
As an exciting pulse is applied to the trigger terminal of a monostable flip-flop 16, the monostable flip-flop 16 outputs a pulse, the width thereof being determined by the resistance value of a timer resistor 17 and the capacity of a timer capacitor 18.
The output of the monostable flip-flop 16 and the output of the comparator 14 are applied to an AND gate 19.
In the above circuit, a loss of synchronism is detected when the output of the AND gate 19 goes to a "H" level when the output of the comparator is reversed, that is, when the current through the exciting coil 11 becomes larger than the fixed reference value.
Though the constant-voltage exciting system has a simple structure and a low price, it is feared that the stepping motor loses synchronism when the stepping motor is rotated at high speed.
To solve the above problem, a constant-current exciting system which controls the current though the exciting coil to a fixed value has already been proposed.
FIG. 2 is a circuit diagram of the constant-current exciting system, and one terminal of the exciting coil 21 is grounded through a control transistor 22 and a resistor for measuring a current 23, and the other terminal thereof is applied to a DC bus V.sub.cc.
An exciting pulse is applied to a chopper circuit 24, and a exciting current measured by the resistor for measuring a current 23 is also fedback to the chopper circuit 24.
Recently, it is required not only to rotate at high speed, but also to surely detect a loss of synchronism when it occurs because a stepping motor is widely used.
It is, however, impossible to detect a loss of synchronism by detecting that the exciting current becomes larger than a fixed reference value, because the exciting current through the exciting coil is controlled to the constant value.